Building an IP Address

Under TCP/IP, every system on a network is assigned a 32-bit IP address. Instead of representing these bytes in hexadecimal, however, they are normally written in the format w.x.y.z, where each variable is replaced with a decimal number between 0 (hex 00) and 255 (hex FF). For example, 192.0.0.12 is a valid IP address. The IP address is further broken down to identify a logical network address and a system. For example, 192.0.0.12 identifies a specific system, "12," within a specific network, "192.0.0." Similarly, address 128.10.20.12 identifies system "20.12" within network "128.10."

You can create an IP address in one of three formats, called classes. Each class breaks the address down into network and host system components differently. The three classes are:

Class A: Uses the format "network.host.host.host," with the network component falling between 0 and 127 (exclusive of 0 and 127), and the host component being greater than 0. For example, in the address 64.0.1.12, "64" identifies the network, and "0.1.12" the host system.

Class B: Uses the format "network.network.host.host," with the first network component falling in the range from 128 through 191 (including 128 and 191), and the host component being greater than 0. For example, in the address 130.101.0.68, "130.101" identifies the network, and "0.68" the host system.

Class C: Uses the format "network.network.network.host," with the first network component falling in the range from 192 through 223 (including 192 and 223), and the host component being greater than 0. For example, in the address 200.1.1.37, "200.1.1" identifies the network, and "37" the host system.

All three formats identify a particular host system within a specific network. A fourth type of address--a Class D address--is used for multicast addresses. Multicast addresses are intended for multiple systems that may reside on different networks; therefore, Class D addresses have no network or host components.

Class D: Uses addresses beginning with a number in the range from 224 through 239 (inclusive).